✔ 最佳答案
sin(2x) = 2sinxcosx is a trigonometric identity called the double angle identity. It was derived from another identity:
sin(x + y) = sinxcosy + cosxsiny
here x and y are two different angles but what if x = y
replace y with x to make all the equations in terms of x, thus the equation becomes:
sin(x + x) = sinxcosx + cosxsinx
note that sinxcosx is also equal to cosxsinx, so simplifying the equation it becomes:
sin(2x) = 2sinxcosx or sin(2x) = 2cosxsinx